Foam structures are known in industry and the number of applications for metallic foam structures is continually increasing. For example, aluminium foam metal having a continuously connected, open celled (reticulated) geometry is available and employed in energy/impact absorbers, heat exchangers and lightweight composite panels.
When used with heat exchangers the high surface to volume ratio allows for a compact design and the high specific stiffness, that is, high strength to weight ratio makes the material useful in aerospace and car applications.
Low-cost aluminium foam panels can be produced by a continuous casting process. The foam is machinable by common aluminium metal working techniques (sawing, drilling, milling) and maybe joined by brazing or adhesive bonding. As previously indicated aluminium foam produced by this method finds application as lightweight cores for sandwich panels and as components in energy absorbing structures.
However, when gas containment vessels are required and in particular when irregular complex shapes are required then metal foams are formed typically by mixing small quantities of a gasifier e.g. titanium nitride with aluminium powder and subjecting the mixture to heat and pressure to form a sintered sheet.
The sintered sheet or a portion thereof is then placed in a mold which is then heated to a higher temperature at which the metal melts and nitrogen is released from the titanium nitride to provide an even dispersion of bubbles.
The hot metal is allowed to solidify and then shock heat treated by dropping it into a cryogen such as liquid nitrogen which causes small fractures to occur between adjacent bubbles so that the mass becomes reticulated. However, by subjecting the mass to a violent heat shock said small fractures may be prone to brittle fracture and also create areas of crack propagation particularly if cyclic loading is applied to the object made from the mass.
It is an aim of the present invention to mitigate against these disadvantages by employing a hot inert gas which is passed through the mass such that thin metal adjacent the fractures melts slightly and is drawn in to a more circular cross-section by surface tension.
According to the present invention a method of making a metal foam object includes the steps of mixing a gasifier with metal powder and subjecting the mixture to an elevated temperature T1 and pressure P1 to form a sintered sheet, placing at least a portion of the sintered sheet in a mold and subjecting the mold to a temperature T2 where T2 is greater than T1 at which the metal melts and the gas is released from the gasifier quenching the metal foam object thus formed by the mold and passing a hot inert gas through the metal foam object.
Preferably, the hot inert gas is argon which is passed through the metal foam object by means of at least one header tube.